1. Field of the Invention
The present invention relates to an over-filled scanner (OFS) type optical scanning apparatus and an image forming apparatus using the same. More specifically, the present invention facilitates image forming apparatuses in which a light beam (laser beam) emitted from a light source device is reflected and deflected with a rotating polygon mirror serving as a light deflector, and a surface to be scanned is scanned with the light beam through an imaging optical system so as to record image information. Such image forming apparatuses are, for example, laser beam printers, digital photocopiers, and multifunctional printers that have an electro-photographic process.
2. Description of the Related Art
An optical scanning apparatus is commonly used as a scanning optical system for laser beam printers, digital photocopiers, multifunctional printers, and so on. Recently, with the spread of these apparatuses, the demand for higher definition and higher speed has been increased.
An over-filled optical system (hereinafter referred to as OFS) meets the two demands, to increase the definition and to increase the speed. The OFS is characterized in that the number of deflecting surfaces of a rotating polygon mirror can be increased without increasing the size of the rotating polygon mirror. If a multi-beam light source that emits M light beams is used as a laser light source (light source device), the speed can be increased M-fold.
When the number of surfaces of the rotating polygon mirror is N, the effective scanning width on a surface to be scanned is W, the focal length of the imaging optical system is f, and the scanning efficiency is duty, there is the following relationship among N, W, f, and duty:f=WN/(4π·duty)According to the above relational expression, the focal length f is proportional to the number of surfaces of the rotating polygon mirror N. Therefore, increasing the number of surfaces causes the optical system to be large.
To solve this problem, various optical scanning apparatuses have been discussed.
An optical scanning apparatus discussed in Japanese Patent Laid-Open No. 2000-267030 (Patent Document 1) is reduced in size because, in the OFS, a converging light beam is incident on the rotating polygon mirror, and imaging is performed at a distance smaller than the focal length of the imaging optical system. Specifically, in the case of a scanning optical system that scans the width in the direction of the short side of A3 size paper (document), whose effective scanning width is 297 mm, when the number of surfaces of the rotating polygon mirror is 12 and the scanning angle is 25.6 degrees, the distance from the deflecting surface to the surface to be scanned is reduced from 437 mm to 305 mm, that is to say, to 68% by causing a converging light beam to be incident on the deflecting surface. In addition, the apparatus of Patent Document 1 is reduced in size by facilitating a tolerance of the fθ performance error up to 0.5% and sacrificing the print-position accuracy in the main scanning direction.
In an optical scanning apparatus discussed in Japanese Patent Laid-Open No. 11-183837 (Patent Document 2), a pair of angled mirrors are disposed between the rotating polygon mirror and the surface to be scanned, and the optical path is folded in the sub-scanning direction, thereby reducing the actual optical-path length to a half of the developed optical-path length.
The optical scanning apparatus of Patent Document 1 has the following problem. Since a converging light beam is incident on the rotating polygon mirror, convergent jitter occurs. In addition, in principle, reduction in the optical-path length causes deterioration in optical performance.
The optical scanning apparatus of Patent Document 2 has the following problem. Since the optical path is folded in the direction of the height of the apparatus, the actual height in the sub-scanning direction increases. In addition, the whole apparatus is complicated (high-cost) due to the increased number of mirror parts.
The OFS is characterized in that the number of surfaces of the rotating polygon mirror can be increased with the size (diameter) of the rotating polygon mirror kept small. In addition, although the diameter is small, the OFS facilitates narrowing down the spot diameter in the main scanning direction because the diameter of the deflected light beam is determined by the width of the deflecting surface (reflecting surface). In the OFS, the number of surfaces has been set to 12 in order to make full use of the characteristics of the multifaceted OFS. In the conventional UFS (under-filled optical system), it can be difficult to set the number of surfaces to 12. However, since the focal length f is proportional to the number N of surfaces as illustrated in the above relational expression, it is inevitable that the apparatus is increased in size. However, if the number of surfaces is reduced, a notable reduction in the diameter of the rotating polygon mirror makes it difficult to manufacture the rotating polygon mirror.